1. Field of the Invention
The present invention relates to a reverse conducting semiconductor device in which an insulated gate bipolar transistor and a free wheeling diode are monolithically formed on a substrate as a single piece and a fabrication method thereof.
2. Description of the Background Art
In recent years, inverter units have been widely used in home electric products, industrial power equipment and the like. An inverter unit is, since it usually employs a commercial power supply (an AC power supply), constituted of: a converter section conducting a forward conversion converting an AC power supply to a DC form once; a smoothing circuit section; and an inverter section inversely converting a DC voltage to an AC voltage. In the inverter section, insulated gate bipolar transistors capable of a high speed switching are mainly used as a main power device. In an inverter unit for power control, a current rating and voltage rating handled by each transistor chip are almost in the range of from several A to several hundreds A and in the range of from several hundreds V to several thousands V, respectively, and in a circuit which is operated by continuously altering gate voltages of insulated gate bipolar transistors (IGBT) with a resistance load, a power, which is a product of a current and a voltage, is liberated in the interior of the transistor as heat. With such construction, the circuit requires a large capacity radiator and a conversion efficiency is also degraded, and since a thermal breakdown results due to rise in temperature of the transistors themselves according to a combination of an operating current and operating voltage, a resistance load circuit is not used in many of cases.
Since in many of cases, a load of an inverter unit is an electric induction machine (a motor as an inductive load), an IGBT usually acts as a switch to repeat an off state and an on state, combined, and to thereby control a power energy. In a case where an inverter circuit is switched with an inductive load, stages of the transistor include a turn-off stage where the transistor alters from an on state to an off state, a turn-on stage where the transistor alters from the off state to the on state and a stage where the transistor stays in the on state, wherein a current flowing into the load and the inductive load are connected to an intermediate potential point between the upper arm and lower arm, a current flowing through the inductive load has both directions, positive and negative and a current flowing into the load is returned from a load connection end to the power supply side at a high potential or caused to flow to the ground side; therefore, a necessity arises for free wheeling diodes, which are used in current circulation of a large current flowing through the inductive load between the load and a close circuit of the arms.
In a conventional inverter circuit including IGBTs and free wheeling diodes, free wheeling diodes for current circulation in anti-parallel connection have been used as a separate chip since an IGBT has no bidirectional current flow capability. In recent years, however, a proposal has been made that diodes for current circulation and IGBTs are integrally built as a single piece for the purpose to realize an inverter unit in smaller size and weight (JP-A No. 2000-200906 and JP-A No. 10-74959).
However, there have been problems that a conventional reverse conducting insulated gate bipolar transistor has an anode and cathode on the same plane, that current flow capabilities in both directions are not at the same level, that since an internal diode shares an element for constituting the insulated gate bipolar transistor, it is difficult to optimize a diode characteristic independently, and that especially a recovery characteristic of an internal diode is lower than that of a diode in a case where an IGBT chip and a free wheeling diode chip are combined as a two-chip device.
Carriers accumulated in the junction end region located in the outer peripheral region of the chip cause current concentration in the boundary region with a cell portion while a recovery action works, resulting in another problem of breakdown of the chip.